生成抽象基类Python

Question

Problem:

I'd like to generate an abstract class with a few method stubs that should be implemented by its sub-classes thereby avoiding any hard-coding.

That is, if I define a class as follows:

>>> class Sample(SomeClass):    
...    ACTIONS = ('run', 'sleep', 'kill')

The above class should always implement the methods run , sleep , and kill .

Attempted solution:

The snippet below works as expected and raises an error when the abstract method run is not defined in subclass FooBar .

>>> class Foo(object):
...     __metaclass__ = ABCMeta
...     
...     @abstractmethod
...     def run(self, *args, **kwargs):        
...         pass    


>>> class FooBar(Foo):
...     def __init__(self, a, b):
...         self.a = a
...         self.b = b

>>> FooBar(1, 2)        
TypeError: Can't instantiate abstract class FooBar with abstract methods run

I was trying to achieve the same by generating the Foo class using type :

>>> Foo2 = type('Foo', (), {'__metaclass__': ABCMeta, 
...                         'run'          : abstractmethod(lambda self, *args, **kwargs: None)})

>>> class FooBar(Foo2):
...     def __init__(self, a, b):
...         self.a = a
...         self.b = b

However, the below call does not raise any error:

>>> FooBar(1, 2)        
<__main__.FooBar at ...>

Noticed that __abstractmethods__ doesn't get defined in Foo2 (comparing vars(Foo) and vars(Foo2) ).

I did find a similar attempt here , but wasn't able to follow why the above wouldn't work and why I can't make use of ABCMeta directly (which defines __new__ in a generic manner).

Answer 1

Realized I could just do this :)

>>> Foo2 = ABCMeta('Foo', (), {'run': abstractmethod(lambda self, *args, **kwargs: None)})

>>> FooBar(1, 2)
TypeError: Can't instantiate abstract class FooBar with abstract methods run